In an optical disk drive apparatus, a seek operation for moving an optical head (and therefore a laser beam) from a current position (or a current track) to a target position (or a target track) across intermediate tracks is performed initially by a coarse seek operation in which the optical head is moved under the direction of a linear position sensor to the neighborhood of the target track at a high velocity. The coarse seek operation is followed with a fine seek operation in which the optical head is moved across remaining tracks one at a time to the target track at a velocity on the order of milliseconds per track. However, this method requires substantial time for the fine seek operation and it becomes difficult to decrease the total seek time. For the above reason, methods of a seek operation performed responsive only to a tracking error signal ("TES") without the linear position sensor have been developed. The chief methods of the seek operation performed by using only the TES are as follows:
FIG. 5 shows a block diagram of an optical disk drive apparatus using a first conventional method. In FIG. 5, the focus control of a beam spot 23 applied to the surface of an optical disk 5 can be performed as follows:
A focusing error signal (FES) sensor 6 detects a focus error and generates a focus error signal. Based on the FES, a focus servo controller 9, a focus voice coil motor ("VCM") driver 10, and a focus actuator 2 perform the position control of an objective lens 24 in the focusing direction so that a focal point of the objective lens 24 remains on the surface of the optical disk.
With respect to the position control of the beam spot 23 in the tracking direction ( that is, in the radial direction ) of the optical disk, the track following operation and the seek operation are separately described. First, in the track following operation, a switcher 11 is switched, by a signal on a seek/track following control line 30 carrying output from a microprocessor and logic 22, to receive an output of a tracking error signal sensor 7 which detects a position of the beam spot 23 relative to a track groove on the optical disk 5 and generates a TES. In response to the TES, a fine servo controller 12, a fine actuator VCM driver 13, and a fine actuator 3 drive the objective lens 24 in the tracking direction so that the position of the beam spot 23 relative to the track groove on the optical disk 5 becomes zero. The beam spot 23 is thus positioned on a current track. Then, a position of the objective lens 24 relative to an optical head 1 is detected by a lens position sensor 8 to generate a relative position error signal; a coarse servo controller 14, a coarse actuator VCM driver 16, and a coarse actuator 4 perform the position control of the optical head 1 in the tracking direction so that the relative position error ("RPE") signal (also called lens position error signal) becomes zero. In this case, a switcher 15 is switched, by a signal on the seek/track following control line 30, to receive an output of the coarse servo controller 14. Thus, the beam spot 23 is positioned on a current track and a position of the optical head 1 is controlled so that the RPE signal becomes zero and the head follows the objective lens 24.
In a seek operation in which the beam spot 23 moves from a current track to a target track, the microprocessor and logic 22 presets a track counter 21 with the number of tracks from the current track to the target track. The seek/track following control line 30 is set to the seek state, the switcher 11 is switched to receive the RPE, and the switcher 15 is switched to receive an output of a velocity comparator 19. Since the switcher 11 is switched to receive the RPE, the objective lens 24 is controlled by the fine servo controller 12, the fine actuator VCM driver 13, and the fine actuator 3 so that a position error of the objective lens 24 relative to the optical head 1 becomes zero. A track crossing signal is generated each time the beam spot 23 crosses a track groove on the optical disk 5 and is detected by a track crossing detecting circuit 20. As the track crossing signals are detected by the track crossing detecting circuit 20, the track counter 21 counts down from its preset value. The contents of the track counter 21 are outputted to a reference velocity generating circuit 18 and which outputs a reference velocity for the remaining tracks to the velocity comparator 19. Simultaneously, a TES is converted by a laser beam track crossing velocity detecting circuit 17 to a signal representative of a velocity of movement of the beam spot 23. The velocity signal outputted from the laser beam track crossing velocity detecting circuit 17 to the velocity comparator 19 where it is compared with the reference velocity from the reference velocity generating circuit 18. A velocity error signal is outputted. Since the switcher 15 is switched to receive the output of the velocity comparator 19, the coarse actuator 4 is driven based on the velocity error signal and the coarse actuator VCM driver, and velocity control is performed so that the velocity of the beam spot 23 follows a reference velocity. When the beam spot 23 reaches the target track, the microprocessor and logic 22 switches the seek/track following control line 30 to the track following state in which the beam spot is under position control.
An example of the laser beam track crossing velocity detecting circuit and exemplary waveforms are shown by FIG. 6 and FIG. 7. The velocity detecting circuit is a frequency/voltage converter. As shown in FIG. 6, the zero crossings of a TES (a) are converted to binary form (b). The binary output is converted, by a monostable multivibrator, to a series of pulses having a constant width interval (c); low-pass filtering is applied to the pulses to obtain velocity information (d). In another method, shown in FIG. 7, the pulse durations of binary output (b) are counted by using a counter (e) and thus obtains velocity information (f) is obtained.
FIG. 8 is a block diagram showing a optical disk drive apparatus using a second conventional method. The focus control of the beam spot 23 irradiating the surface of the optical disk 5 is performed by the FES sensor 6, the focus servo controller 9, the focus VCM driver 10, and the focus actuator 2 in the same way as the first conventional method described above. The track following operation of the beam spot 23 and the position control of the optical head 1 are also performed in the same way as the first conventional method. That is, a switcher 101 is switched to receive the TES and the track following control of the beam spot is performed, based on the TES, by the TES sensor 7, the fine servo controller 12, the fine actuator VCM driver 13, and the fine actuator 3; the position control of the optical head 1 is performed, based on an RPE signal, by the lens position sensor 8, the coarse servo controller 14, the coarse actuator VCM driver 16, and the coarse actuator 4.
In the seek operation in which the beam spot 23 moves from a current track to a target track, the microprocessor and logic 22 presets a track counter 21 with the number of tracks from the current track to the target track. The seek/track following control line 30 is set to transmit the seek state and the switcher 101 is switched to receive a position error signal ("PES") from an integrator 105. A track crossing signal is generated each time the beam spot 23 crosses a track groove on the optical disk 5 and is detected by the track crossing signal detecting circuit 20. As the track crossing signals are detected by the track crossing detecting circuit 20, the track counter 21 counts down from its preset value. The contents of the track counter 21 are outputted to the reference velocity generating circuit 18 which outputs a reference velocity for the remaining tracks to a velocity comparator 104. Simultaneously, a TES is converted, by a differentiator 102 and a rectifier 103, to a differential tracking position signal representative of a velocity of movement of the beam spot 23. This signal is compared with the reference velocity by the velocity comparator 104. The output of the velocity comparator 104 is integrated by the integrator 105 and outputted as a position error signal. The fine actuator 3 is driven, based on the PES, by the fine servo controller 12 and the fine actuator VCM driver 13, and velocity control is performed so that the velocity of the beam spot 23 follows a reference velocity. The optical head 1 moves in such a way that it follows the positions of the objective lens 24 so that a lens position error becomes zero, as described above. When the beam spot 23 reaches the target track, the microprocessor and logic 22 switches the seek/track following control line 30 to the track following state and the beam spot is placed under position control.
In the following, the operations of the differentiator 102, the rectifier 103, the velocity comparator 104 and the integrator 105 are described by reference to the waveforms illustrated in FIG. 9. A TES is converted, by the differentiator 102 and the rectifier 103, to a differential tracking position signal representative of the velocity movement of the beam spot 23 and compared with a reference velocity associated with the number of remaining tracks by the velocity comparator 104. The output of the velocity comparator 104 is processed by the integrator 105 and outputted as a position error signal (PES). Since the velocity of the beam spot 23 is low immediately after the seek operation is started, a PES having a large amplitude is inputted to the fine servo controller 12 to accelerate the fine actuator 3. If a significantly low reference velocity is provided when the head is in the neighborhood of a target track, indicating that the velocity of the beam spot reaches has approached a track following controllable velocity, the velocity control is performed even before a target track center is reached, and therefore control for positioning on a target track can be exactly performed.
The first method of the prior art has been generally employed with a magnetic disk drive apparatus. However, the method cannot set a sufficiently high controllable frequency of the coarse actuator 4 in a optical disk drive apparatus which has a larger and heavier head than that of a magnetic disk drive apparatus. Therefore, the velocity of the coarse actuator 4 cannot be precisely controlled when the beam spot 23 is in the neighborhood of a target track and an attempt to shift to the track following control fails.
Frequencies of a TES range from 1 kHz to hundreds of kilohertz in the seek operation and it is desirable to increase the frequency range of operation of the laser beam track crossing velocity detecting circuit in order to realize high-speed track access. And, in some cases it is necessary to switch among controlling circuits with plural bandwidths which increases circuit complexity. Further, the method for detecting a velocity described in the first method detects the velocity only after the crossing a track, and therefore the velocity control would become delayed immediately before a target track.
The second method of seek control can provide much the same control as is provided by the tracking control when the beam spot 23 is in the neighborhood of the target track, since the fine actuator 3 is primarily controlled by a PES corresponding to a position error. Therefore, fine velocity control and position control become possible and reliable shifting to the track following control is realizable. However, in the second method, the fine actuator 3 is primarily controlled and the seek operation is secondarily controlled in such a way that the coarse actuator 4 controlled by an RPE signal is under the following operation. Therefore, the acceleration and deceleration of the coarse actuator 4 are delayed and it becomes difficult to make the most of the ability of acceleration or deceleration in the coarse actuator 4. Further, in the second method, as in the first method, frequencies of the TES range from 1 kHz to hundreds of kilohertz during the seek operation, and therefore it is necessary to increase the frequency range of operation of the differentiator 102 and the rectifier 103 in order to realize high-speed track access. In some cases it is necessary to switch among controlling circuits with plural bandwidths, which increases the circuit complexity.